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It's quite simple, in order to calculate checksum numbers for data in order to later compare them between master and slave, those calculations have to be done separately on master and slave. With ROW binary format, the slave is only applying row changes from master, so the checksum table would be the same. Yet we need statements replicated to slaves, not data. But that's just per pt-table-checksum session setting, all the rest traffic goes via globally set binlog format.

Just to be clear, pt-table-checksum is setting its SESSION level binlog_format to STATEMENT and not GLOBAL? I noticed that the global setting is still MIXED when running "SHOW GLOBAL VARIABLES LIKE 'binlog_format';" while pt-table-checksum is running "nohup pt-table-checksum --no-check-binlog-format --empty-replicate-table > /dev/null 2>&1 &"

Why is this? Is this explained in more detail somewhere? What is the issue exactly? Because filling up the log with these warnings doesn't seem like correct behaviour.

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Well I understand what you are saying Przemek. I understand that it has to replicate the statements to the slaves, in order to re-run them there, and not the data. That is fine. But why it doesn't run when you have a MIXED binlog_format on the master and the slaves? It is asking me to put the --no-check-binlog-format option on pt-table-checksum script. When I put the options it does run and does checksum both server and slaves which are in MIXED mode. But unfortunately it requires that the user has the SUPER privilege. Because it needs that to set the binlog_format to STATEMENT at run time. Is this really necessary?